1. Field of the Invention
This invention relates generally to a system for stabilizing and controlling a hoisted load. The invention relates more specifically to a system for stabilizing and controlling in six degrees of freedom the movement of a hoisted load. The invention relates even more specifically to a system which can both be adapted to existing single point lift mechanisms, and constrain the load in all six degrees of freedom.
2. Description of Related Art
As discussed in U.S. Pat. No. 4,883,184, lifting platforms are commonly attached to cranes, such as overhead tower-type cranes having a horizontal boom and boom-type cranes having a diagonal boom. Applications for these lifting platforms can include transporting cargo on and off ships, and relocating necessary equipment and materials on a construction site.
The potential motions of a hoisted object can best be envisioned by means of a Cartesian coordinate system in which the z-axis is in the vertical direction, and the x and y axes form the horizontal plane. The rotation of the hoisted object about the z-axis is therefore defined as yaw, rotation about the x-axis is defined as pitch, and rotation about the y-axis is defined as roll.
In typical load transporting applications, a crane will have a single lifting cable. In these applications, the lifting cable is stable only in the z direction. Under any external influence from the sides, the load will either roll, pitch, or yaw, or will sway in the x and y directions.
The prior art has long recognized the need to compensate for these motions, and as a result, various conventional devices exist for attempting to stabilize a hoisted load. For example, U.S. Pat. No. 4,171,053 describes a crane for overcoming the undesirable effects of cargo pendulation. The crane consists of conventional booms, vertical hoist lines, and a hook member for engaging the cargo to be lifted and lowered. The crane also consists of a horizontal beam located at the base of the boom. The major portion of the hoist lines remains in substantially a vertical plane as a result of lines which extend from a guide means at the bottom of the hoist lines to the horizontal beam.
U.S. Pat. No. 4,883,184 describes a cable arrangement and lifting platform for lifting a load in a stabilized manner. The lifting platform secures loads to a securing device and the platform is able to be suspended from a crane by an attachment carriage. The attachment carriage includes a cable winch onto which six cables suspend and attach to the lifting platform. The attachment carriage also includes cable guides which guide the six cables away from the winch in three cable pairs, preferably equidistantly-spaced. In order to secure the cables to the lifting platform, the platform includes an attachment frame having three cable attachment points, preferably spaced equidistantly apart with respect to each other. The lifting platform helps stabilize the lifting of loads by sensing the load""s imbalance relative to the center of mass of the platform and repositioning the load to correct for the imbalance.
U.S. Pat. No. 4,932,541 describes a stabilized cargo-handling system using means for stabilizing suspended cargo in all six degrees of freedom using six individually controlled cables in tension in a kinematic arrangement. Inertial and distance sensors, coupled with high-performance cable drives, provide the means to control the multi-cabled crane automatically. The distance sensors are used to track the target container or lighter vessel during the pickup and setdown modes of operation; the inertial sensors are used to prevent pendulation during transfer of the cargo from the seagoing cargo ship to the vicinity of the receiving lighter.
U.S. Pat. No. 5,507,596 describes an underwater work platform supported by a plurality of cables connected between a support structure and the work platform. Motions of the support structure in the body of water are sensed, and the length of the cables is adjusted in response to the sensed motion of the support structure so that the work platform can be maintained in a stationary position even when the support structure is subjected to wave forces and currents.
In the late 1980""s the National Institute of Standards and Technology (xe2x80x9cNISTxe2x80x9d) developed a concept known as RoboCrane based on a Stewart platform geometry parallel link manipulator, but which uses cables as the parallel links and winches as the actuators.
NIST also developed a version of the RoboCrane known as TETRA for testing long cable suspensions. TETRA includes winches mounted on the work platform as opposed to the supporting structure. TETRA""s relatively light duty winch cables are used to augment existing heavy duty lift equipment (such as cranes) by attaching to the suspended load and then using RoboCrane control programs to provide intuitive load control in six degrees of freedom.
Single point lift mechanisms, such as boom-type cranes, typically include a base, a boom, and a heavy duty hoist system including a winch and block and tackle. As indicated above, however, load pendulation is a basic problem typical of such cranes since they can only control the vertical axis. Attempts at controlling load pendulation have included control programs that maneuver the lift point to stay above the load. Others attempts have included the use of reeving (like the RoboCrane) and vertical motion compensation.
A vessel known as a Tactical Auxiliary Crane Ship (xe2x80x9cT-ACSxe2x80x9d) includes a system called the Rider Block Tagline System (xe2x80x9cRBTSxe2x80x9d) that attempts to stabilize a load by pulling on taglines to prevent large pendulations. The RBTS, however, affords limited control of the spreader/cargo sway, and no rotational control of the spreader/cargo. Additionally, the RBTS introduces complex load motions that are difficult to dampen, so that operators often disable the system. Furthermore, the RBTS hinders performance and safety as a result of depth perception and line of sight occlusion, and requires the presence of ground personnel with tag lines in hazardous areas to guide the load. Routine RBTS operations, therefore, require precision boom control and a highly trained operator. Finally, the RBTS does not control the load in all six degrees of freedom.
While the aforementioned conventional devices may therefore provide varying degrees of control of a hoisted load, not all of these devices can control all six degrees of freedom, and none can both be adapted to existing single point lift mechanisms, and constrain the load in all six degrees of freedom, thus satisfying a long-felt need in this environment.
It is an object of the present invention to provide a system which can both be adapted to existing single point lift mechanisms, and constrain a hoisted load in all six degrees of freedom.
Accordingly, the present invention advantageously relates to a system for stabilizing and controlling in six degrees of freedom the movement of a hoisted load. The system comprises a suspension point, an assembly, a lateral tension lines member, and a control system. In a first embodiment, the assembly comprises a first platform for positioning the assembly; a second platform disposed below the first platform; first, second, third, fourth, fifth, and sixth, control lines having a first end and a second end, with the control lines disposed between first platform and the second platform; an assembly hoist, which comprises first, second, and third assembly hoist lines in communication with a corresponding one of each of first, second, and third assembly hoist line length adjusters; and a load hoist which comprises a load hoist line and a load hoist connector, with the load hoist line in communication with a load hoist line length adjuster.
The first platform comprises a first platform upper surface, a first platform lower surface, a first platform outer edge, load hoist line guides in slidable communication with the load hoist line, and a plurality of lateral tension line connectors for engaging a plurality of lateral tension lines for providing lateral tension to the first platform, with the plurality of lateral tension lines in communication with a corresponding one of a plurality of lateral tension line length adjusters.
The first platform upper surface comprises first, second, and third assembly hoist line connectors for removably engaging a corresponding one of each of first, second, and third assembly hoist lines. The first platform lower surface comprises first, second, and third control line end connector pairs for removably engaging the first end of each of the first, second, third, fourth, fifth, and sixth control lines. The control line end connector pairs are arranged in a substantially triangular configuration on the first platform lower surface, with first control line end connector pair engaging the first and sixth control lines, the second control line end connector pair engaging the second and third control lines, and the third control line end connector pair engaging the fourth and fifth control lines.
The second platform comprises a second platform upper surface, a second platform lower surface, and a second platform outer edge. The second platform upper surface comprises first, second, third, fourth, fifth, and sixth control line length adjusters for adjusting the length of each of the corresponding first, second, third, fourth, fifth, and sixth control lines. The control line length adjusters are arranged in first, second, and third control line length adjuster pairs in a substantially triangular configuration on the second platform upper surface, and are in communication with the second end of a corresponding one of the first, second, third, fourth, fifth, and sixth control lines. The first control line length adjuster pair comprises first and sixth control line length adjusters, the second control line length adjuster pair comprises second and third control line length adjusters, and the third control line length adjuster pair comprises fourth and fifth control line length adjusters. The second platform upper surface comprises a load hoist receiver for removably receiving the load hoist connector.
The substantially triangular configuration of control line length adjuster pairs is oriented relative to the substantially triangular configuration of control line end connector pairs such that each vertex of the control line length adjuster pairs configuration is at a position diametrically opposed to a side of the control line length adjuster pairs configuration.
The control system comprises first, second, third, fourth, fifth, and sixth tension-sensors in communication with a system controller, with each of the first, second, third, fourth, fifth, and sixth tension sensors associated with a corresponding one of each of the first, second, third, fourth, fifth, and sixth control lines for determining a tension of each of the control lines. A plurality of lateral tension line tension sensors are in communication with the system controller, with each of the plurality of lateral tension line tension sensors associated with a corresponding one of each of the plurality of lateral tension lines for determining a tension of each of the lateral tension lines.
The control system comprises at least one motion sensor for sensing motion of the load, with the motion sensor in communication with the system controller, and at least one proximity sensor for sensing the proximity of the assembly to an objective position, with the proximity sensor also in communication with said system controller.
The control system facilitates stabilization and control of the load by adjusting the position of any one or more of the plurality of lateral tension lines and/or of any one or more of the first, second, third, fourth, fifth, and sixth control lines. The load can also be stabilized and controlled by adjusting the tension in any one or more of the plurality of lateral tension lines and/or in any one or more of the first, second, third, fourth, fifth, and sixth control lines. The load can also be stabilized and controlled with simultaneous position and tension control. The control system comprises an intuitive multi-axis joystick and a computer, thus facilitating manual control, automatic control, or a combination of manual and automatic control.
The present invention, therefore, utilizes a first platform instead of the rider block of the RBTS, and employs additional lateral lines to constrain the yaw of the first platform. The invention also adds the unique RoboCrane capabilities, such as the control cable configuration and kinematic control, by virtue of the second platform suspended from the first platform. The system, therefore, solves the load pendulation problem by providing a suspended, constrained assembly to resist forces and torques incurred from the environment and/or induced by the crane. So long as the lines are all in tension, the load is kinematically constrained with a mechanical stiffness determined by the elasticity of the lines and the suspended load.
Advantages associated with the system include the ability not only to stabilize and control a load while it is being lifted or lowered, but to hold a load stationary in a suspended position, as is desirable when the load is a tool. Advantages associated with the various embodiments of the system include both its ready adaptation to existing single point lift mechanisms, its relatively light weight, and its flexibility, ease, and precision of operation.